protocols/screencopy: Make frame/session send stopped/fail on drop

Previously, `Frame` was stored in KMS frame udata, but in some cases the
udata was dropped without a capture happening, and `Frame` did not
implement `Drop`, so `fail` was never sent.

Instead, rename `DropableFrame` to `Frame` and `Frame` to `FrameRef`, so
we can have a single instance of `Frame`, that will send `fail` on drop.
This guarantees either `.success` or `.fail` are send, as long as its
not leaked.

This seems to fix https://github.com/pop-os/cosmic-comp/issues/1305.
xdg-desktop-portal-cosmic prints an error, buy retries (as it should for
an `Unknown` error; though maybe there should be a retry limit) and the
session continues working.

(Not sure if it should be sending `failed`, or queing it with the next
frame so it can send `success` to the client, but this works and is
desirable as a failsafe anyway.)

`Session` and `CursorSession` are similiarly updated.

`.fail()`, `.success()`, and `.stop()` now consume
`Frame`/`Session`/`CursorSession`. So to stop a session, it is now
necessary to call `.remove_session()`, but then simply dropping with
send `.stop()`.

Factoring out some `Request::Capture` handling into a `capture_frame`
function seems to clean up error handling and such a bit.

src/backend/kms/device.rs

@@ -5,7 +5,7 @@ use crate::{
     config::{AdaptiveSync, EdidProduct, OutputConfig, OutputState, ScreenFilter},
     shell::Shell,
     utils::prelude::*,
-    wayland::protocols::screencopy::Frame as ScreencopyFrame,
+    wayland::protocols::screencopy::Frame,
 };
 
 use anyhow::{Context, Result};
@@ -65,7 +65,7 @@ pub type GbmDrmOutputManager = DrmOutputManager<
     GbmFramebufferExporter<DrmDeviceFd>,
     Option<(
         OutputPresentationFeedback,
-        Receiver<(ScreencopyFrame, Vec<Rectangle<i32, BufferCoords>>)>,
+        Receiver<(Frame, Vec<Rectangle<i32, BufferCoords>>)>,
         Duration,
     )>,
     DrmDeviceFd,

src/backend/kms/surface/mod.rs

@@ -13,7 +13,7 @@ use crate::{
     wayland::{
         handlers::screencopy::{submit_buffer, FrameHolder, SessionData},
         protocols::screencopy::{
-            FailureReason, Frame as ScreencopyFrame, Session as ScreencopySession,
+            FailureReason, Frame as ScreencopyFrame, SessionRef as ScreencopySessionRef,
         },
     },
 };
@@ -1024,7 +1024,7 @@ impl SurfaceThreadState {
         // so let's collect everything we need for screencopy now
         let mut has_cursor_mode_none = false;
         let frames: Vec<(
-            ScreencopySession,
+            ScreencopySessionRef,
             ScreencopyFrame,
             Result<(Option<Vec<Rectangle<i32, Physical>>>, RenderElementStates), OutputNoMode>,
         )> = self